gtk: Add suspended window state

This is implemented using a new xdg_toplevel `suspended` state, and is
meant for allowing applications to know when they can stop doing
unnecessary work and thus save power.

In the other backends, the `suspended` state is set at the same time as
`minimized` as it's the closest there is to traditional windowing
systems.

tests/meson.build

@@ -103,6 +103,7 @@ gtk_tests = [
   ['teststack'],
   ['testrevealer'],
   ['testrevealer2'],
+  ['testsuspended'],
   ['testwindowsize'],
   ['testpopover'],
   ['listmodel'],

tests/testgtk.c

@@ -4542,6 +4542,8 @@ surface_state_callback (GdkSurface  *window,
   msg = g_strconcat ((const char *)g_object_get_data (G_OBJECT (label), "title"), ": ",
                      (new_state & GDK_TOPLEVEL_STATE_MINIMIZED) ?
                      "minimized" : "not minimized", ", ",
+                     (new_state & GDK_TOPLEVEL_STATE_SUSPENDED) ?
+                     "suspended" : "not suspended", ", ",
                      (new_state & GDK_TOPLEVEL_STATE_STICKY) ?
                      "sticky" : "not sticky", ", ",
                      (new_state & GDK_TOPLEVEL_STATE_MAXIMIZED) ?

tests/testsuspended.c

@@ -0,0 +1,46 @@
+#include <gtk/gtk.h>
+
+static void
+quit_cb (GtkWidget *widget,
+         gpointer   data)
+{
+  gboolean *done = data;
+
+  *done = TRUE;
+
+  g_main_context_wakeup (NULL);
+}
+
+static void
+report_suspended_state (GtkWindow *window)
+{
+  g_print ("Window is %s\n",
+           gtk_window_is_suspended (window) ? "suspended" : "active");
+}
+
+static void
+suspended_changed_cb (GtkWindow *window)
+{
+  report_suspended_state (window);
+}
+
+int main (int argc, char *argv[])
+{
+  GtkWidget *window;
+  gboolean done = FALSE;
+
+  gtk_init ();
+
+  window = gtk_window_new ();
+  gtk_window_set_default_size (GTK_WINDOW (window), 200, 200);
+  g_signal_connect (window, "destroy", G_CALLBACK (quit_cb), &done);
+  g_signal_connect (window, "notify::suspended",
+                    G_CALLBACK (suspended_changed_cb), &done);
+  gtk_window_present (GTK_WINDOW (window));
+  report_suspended_state (GTK_WINDOW (window));
+
+  while (!done)
+    g_main_context_iteration (NULL, TRUE);
+
+  return EXIT_SUCCESS;
+}